1. Field of the Invention
This invention concerns a frequency modulated signal transmitter, and particularly to a transmitter used in such mobile communications apparatuses as portable telephones and automobile telephones and to a suitable frequency modulation method for application therein.
2. Description of the Related Art
In recent years, with the developments being made in automobile telephones (car phones) and portable telephones, radio communications devices for that purpose are proliferating. These communications devices modulate a carrier wave using the call voice signal as the modulating signal to produce a frequency modulated signal (FM signal), and use this FM signal to conduct communications between a base station and the mobile terminals. Portable telephones and the like which use analog modes are equipped with circuits for transmitting voice signals in the form of FM signals over radio channels. Circuitry has been proposed for such circuits, configured to prevent degradation in the S/N (signal-to-noise ratio) for weak voice signals sent by radio.
FIG. 19 is a block diagram of a conventional FM radio transmitter. This radio transmitter comprises a voice processing circuit 1 and a frequency modulating (FM) circuit 7. The voice processing circuit 1 contains circuitry for signal-processing voice inputs that are input via input terminals. The FM circuit 7 contains circuitry that modulates the frequency of a carrier wave with the voice signal that is output by the voice processing circuit 1 and signal-processed, to generate and output the FM signal.
The voice processing circuit 1 comprises a bandpass filter (BPF) 2, compressor 3, a preemphasis circuit 4, limiter circuit 5, and lowpass filter (LPF) 6. The BPF 2 filters the speech input and outputs a voice signal that is within a certain frequency band. The compressor 3 takes the voice signal from the BPF 2 and compresses it at a 2:1 compression ratio (the ratio between the compressor's input signal and output signal) centered on some reference value, and outputs the compressed signal. For the reference value, in this case, a signal level value is set at which, when the voice signal from the BPF 2 passes through the compressor 3 where it is compressed and output, out of the signal levels of the output signals, a signal is output without being compressed. The preemphasis circuit 4 outputs the output signal from the compressor 3 after emphasizing its higher-level components. The limiter circuit 5 outputs the high-level emphasized signal after limiting its amplitude. The LPF 6 is a lowpass filter that removes the harmonic waves components from the amplitude-limited signal. The signal from which the harmonic waves have been removed by this filter is then output.
With the conventional FM radio transmitter configured in this manner, weak voice signals are enhanced by compressing the voice signal with the compressor 3. The preemphasis circuit 4 emphasizes the high levels of the voice signal compressed by the compressor circuit 3. This high-level emphasized voice signal then passes sequentially through the limiter circuit 5 and LPF 6, and is input as the modulating signal to the FM modulating circuit 7. This circuit 7 outputs a frequency modulated signal (FM signal). By processing the signals in this manner, S/N ratio degradation in weak voice signals carried as radio signals can be prevented.
The FM signal originating from this transmitter is received by a radio receiver (not shown in the drawings). At the radio receiver, the received FM signal is FM demodulated. Then the high levels in the demodulated signal are restored in a deemphasis circuit. After that, the signal to which the higher levels have been restored is restored to the original voice signal by an expander, at a 1:2 expansion ratio, referencing a reference level chosen to correspond with the sending end.
As described in the foregoing, in the conventional FM radio transmitter, the voice signal compressed by the compressor 3 is FM modulated with a certain level of modulation by the FM modulator circuit 7.
Let it be assumed that the reference value for the output of the compressor 3 is -20 dBv. Now, when this -20 dBv signal is modulated with a frequency deviation of .+-.1.5 kHz, the necessary voice level range is, in the Narrow Advanced Mobile Phone Service (NAMPS) system, from .+-.5.0 kHz to .+-.267 kHz. That being the case, the carrier wave must be modulated in the FM modulator circuit by a modulating signal, according to the output from the compressor 3, and then transmitted.
Now, if one follows the standards of the AMPS (TIA/EIAIS-19B) system (where AMPS is the Advanced Mobile Phone Service) and the NAMPS system, the maximum frequency deviation that is FM modulated in response to the compressor output signal level must exhibit linear proportionality.
However, FM noise is generated in the FM modulator circuitry, so that an FM noise component is added to the voice signal. The maximum frequency deviation becomes larger by the size of this added noise component. As a consequence, the level of the modulating signal of the output signal from the compressor 3 and the level of frequency modulation after FM modulation can exceed the standards for FM modulation level relative to signal input level as established in the NAMPS and AMPS systems.
FIG. 20 is a graph for describing maximum frequency deviation when FM modulating in response to a compressor output signal level. The level of the signal output from the compressor (in dBv) is plotted on the horizontal axis, and the maximum frequency deviation (in Hz) is plotted on the vertical axis. In this graph in FIG. 20, the FM noise (.+-.50 Hz) is shown superimposed on the output signal level. In FIG. 20, the straight line a1 represents the maximum frequency deviation vs. the compressor 3 output signal level reference characteristic. The output signal level and the maximum frequency deviation ordinarily are directly proportional. On either side of the straight line a1 are two straight lines b1 and c1 that are both parallel to the line a1. The respective intervals between these straight lines represent standard value widths for the output signal level from the compressor 3 as based on the U.S. TIA/EIA standard IS-90 (Recommended Minimum Standard for 800 MHz Dual-Mode Narrow Band Analog Cellular Subscriber Units). The straight line b1 exhibits upper and lower limiting values for the standard value fluctuation width from below the reference level (in this case, -20 dBv (1.5 kHz peak deviation) to -35 dBv (267 Hz peak deviation)), the fluctuation width of which is .+-.1 dB. Straight line c1 exhibits upper and lower limiting values for the standard value fluctuation width from below the reference level (in this case, -20 dBv (1.5 kHz peak deviation) to -9.54 dBv (5 kHz peak deviation)), the fluctuation width of which is .+-.0.5 dB.
Now, the FM noise components actually generated by the FM modulator circuitry comprise, for example, power supply fluctuations in the FM modulator circuitry and an oscillation frequency fluctuation component caused by hiss noise. Curve d1 in FIG. 20 represents the characteristic when this FM noise (.+-.50 Hz) is superimposed on the output signal from the compressor 3. As may be understood from FIG. 20, when such FM noise is superimposed, if the compressor 3 output signal level is lower than around -30 dBv, then this output signal level and the maximum frequency deviation diverge sharply from their proportional relationship.
Let it be now assumed that the compressor output signal level is -35 dBv, and that FM noise (+50 Hz) is superimposed on this output signal. We can see from FIG. 20 that, in this case, the maximum frequency deviation resulting from the output signal from the compressor 3 will be 267 Hz. When FM noise components such as described above become superimposed on the maximum frequency deviation resulting from this output signal, the maximum frequency deviation for the signal output from the FM modulator circuit will be 267 Hz.+-.50 Hz=317 Hz or 217 Hz. These values exceed the fluctuation standard .+-.1 dB (267 Hz.+-.32.75 Hz) limiting values that are below the compressor fluctuation reference level (-20 dBv) which is the NAMPS (TIA/EIA IS-90) modulation standard.
When FM noise is superimposed on a voice signal, what has been done in the past, in general, is to lower the radio FM noise level and surrounding circuit noise level in order to eliminate the problems caused by FM noise. When this procedure is implemented, however, in a frequency modulated signal transmitter comprised of a combination of various circuits, it is exceedingly difficult to reduce FM noise down to a level at which, in practice, it poses no problem.
Accordingly, an object of the present invention is to provide a frequency modulated signal transmitter wherewith the effects of FM noise components and surrounding circuit noise are suppressed, even if the level of frequency modulation after FM modulation is low, and wherewith the linearity of the FM modulating signal can preserved.